1. Field of the Invention
The present invention relates, particularly for gear cutting machines, to a control circuit providing a positive control of machine tools by means of a circuit provision for avoiding damages such as tool damages, workpiece damages and/or machine damages resulting from power failures or other emergency shut-offs.
2. Brief Description of the Background of the Invention Including Prior Art
Tool machines with positive electronic controls an even wider application due to their flexibility in use with respect to the simpler adjustment to various assortments of workpieces and blanks, made possible with a control program, and without the time consuming changing of change wheels.
Besides the saving on preparation and completion times such machines can also be employed in manufacturing systems controlled by computers.
The drives necessary for electronically controlled machines of this kind (tool drive, workpiece drive and feed drive) are structurally fully independent one from the other and have the advantages of a direct constructive coordination very close to the work place. The disadvantage of the separate drives is the fact that they exhibit a finely differentiated run down behavior in case of interruptions such as for example power outages, drive failures, computer breakdowns, based on the very different moments of inertia of the components.
In the special case of a hobbing cutter the tool drive has a much larger moment of inertia as compared with the workpiece drive. During regular work the tool drive with a large moment of inertia provides the advantage of balancing the variations in the torque caused by interruptions in the cutting process. The tool drive therefor operates in the electronic positive control system as a master drive. The workpiece drive has a comparatively small flywheel effect and acts in an electronic control system as a controlled follower drive in the position to follow and to balance deviations generated by variations in the load and the like from the preset forced run ratio. The speed at which these variations of the load are recognized by the digital control and transformed into a correction of the desired value for the follower drive depends very much on the pulse frequency of the control. Therefore, in order to guarantee satisfactory production qualities very high pulse frequences (T 0.3 ms) are required resulting in high expenditures. For this reason, the realization of such a control is not considered with present day microprocessors having limited speed of operation.
Positive control devices are also known with a digital and an analog branch where a large dynamics is achieved despite a limited work speed of the digital branch. Upon a change in the transmission ratio both the analog as well as the digital transmission ratio controls both branches to be correspondingly changed, which is not easy in cases where high precission is required as in the hob cutting, for example. Therefore, frequency-voltage converters are employed for generation of an analog follower value whereby the adjustment of the transmission ratio has to be performed only in the digital branch. Thus upon failure of the digital branch also the adjustment of analog values is interrupted.
Upon a failure or outage of the power supply considerable damages are to be expected due to the varying run down times. The initially slightly reduced speed of rotation of the tool versus the nearly immediate braking of the workpiece caused by its small effective moments of inertia and the relatively large friction in the guide of the work results in a tangential penetration of the tool into the workpiece, that is into the teeth profiles of the gear wheel to be produced. Thus a destruction of the teeth of the workpiece engaged with the hob cutter results. The hob cutter is considerably braked by the continuously increasing depth of cut and chip thickness and can also be damaged in the process, that is a breaking out of the cutting edges or even a breaking of the hobbing cutter can occur.
If the energy of the slowing down tool drive is sufficient and the hobbing cutter is sufficiently stable, then also a complete shear of the hobbing cutter can occur. On the other hand, in cases of large machines the tool can penetrate to such a depth into the workpiece that the resulting forces render unuseable the machine including the bearing of the tool.
No provisions are made at the known controlled gear cutting machines which would avoid disturbances in case of a power outage or a computer breakdown. For the smaller and medium size gear cutting machines representing sizes for which electronic control provisions have been produced, the existing danger was accepted and in the case of a disturbance only the work piece became a reject, but based on the relatively small milling forces no machine damages occurred and only in exceptional cases did the tool break.
In equipping large gear cutting machines with an electronic controls, decisive economic disadvantages result in case of a disturbance, such that safety measures are required. In machines of this kind the costs of the tool are very high and in large workpieces there are invested time consuming premilling procedures requiring high production and material costs. For this reason, in the gear cutting of premilled large workpieces rejects have to be avoided.
In the case of turning machines having separate drives for the main spindle and the feed of the tool, but which operate in a certain relationship relative to each other via a control electronics such as, for example in thread cutting machines, there is already known a tool withdrawal provision as taught in German publication DE-OS No. 25 42 017, where upon interruption of the electrical energy the tool is withdrawn in order to avoid rejects in the workpieces being milled.
Such a tool withdrawal provision is not suitable for preventing the above mentioned possible damages in positively controlled gear cutting machines for, since in the recognition process of the power failure such as power outage or computer breakdown, an excessively long dead time occurs, which does not eliminate an endangering of the tool, the workpiece or the machine.
A further known possibility of a rapid withdrawal of the tool provides a fold-down tool support, which is tilted away upon a power failure and thereby removes the tool from cutting. This solution has to remain out of consideration in view of the fact that relieving of functionally important building modules from the flux of force is impossible for stability reasons, particularly at machines operating with large milling forces during rough-cutting.